1. Field of the Invention
The present invention relates to a lithographic apparatus and device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The use of an array of LEDs as a source of radiation in a lithographic apparatus has been proposed. In particular, the use of an array of LEDs has been proposed as a radiation source for providing 365 nm (I-line) radiation. However, the peak intensity of an emission spectrum of an LED is known to drift, or in other words fluctuate. Such drift or fluctuation can occur for one of a number of reasons. For example, such drift or fluctuation can occur due to a temperature change of the LED, a change in the current with which the LED is driven, and/or general degradation over time of the LED. Furthermore, in an array of LEDs the wavelength or frequency at which the peak intensity of the emission spectrum occurs may vary between LEDs in the array. As a result, the bandwidth of a radiation source including an array of LEDs will be larger then the bandwidth of an individual LED. If the emission spectra of LEDs within the array are not consistent, there may be associated negative effects on the imaging quality of patterns applied to a substrate. Such negative effects could arise since the lithographic apparatus may be configured to receive, transmit, reflect, etc radiation having a specific wavelength. Therefore, a change in the wavelength at which the peak emission occurs from the array of LEDs can lead to degradation in the reception, transmission, reflection, etc. of the radiation in the lithographic apparatus. A change in the wavelength at which the peak emission occurs from the array of LEDs may also affect the application of patterns to a later of radiation sensitive material, since such material may only be sensitive (e.g., photoreactive) to radiation of a specific wavelength or narrow range of wavelengths.
It is desirable to provide, for example, a lithographic apparatus and method that obviates or mitigates one or more of the problems of the prior art referred to above.